1. Field of Invention
The present invention relates generally to the field of regulating linear motors. More specifically, the present invention is related to hot spot sensoring control of linear motors.
2. Discussion of Prior Art
Vector control techniques used to regulate the motion of linear motor shuttles for high performance applications utilize position sensors so that the location of the moving shuttle is known and current sensors in order to regulate the currents of the motor. However, as in the case of rotary motor systems, the position sensors are a more fragile part of the system and their failure can render the controller useless. For long linear motors, linear position sensors need to extend along the entire length of the track, making them even more problematic. They are subject to environmental stresses and failures, as well as greatly increasing the cost and complexity of the control system. Because of their relatively large air gap and because of end effects, extension of mechanically sensorless control techniques used on rotary motors to the linear motor has been met with only limited success.
There are many types of linear motor systems including synchronous linear motors, switched reluctance linear motors, permanent magnet linear motors, and linear induction motors. The Hot Spot Sensoring Controller is applicable to all of these but the discussion here will focus on the linear induction motor (LIM).
FIG. 1 shows a block diagram of a typical linear induction motor system with a series of stationary stator coils 102, 104, 106, 108 called motor blocks and a moving shuttle 110 which is analogous to the rotor of a rotary induction motor. Shuttle 110 is often composed of a layer of aluminum and a layer of steel or embedded conductive bars in a steel back plate. The voltages are applied to the stator coils 102, 104, 106, 108 which generate currents in the conductors and ultimately generate electromagnetic force that propels shuttle 110 along the track. Force is generated on shuttle 110 only by stator blocks 102, 104, 106, 108 that are energized below shuttle 110. Stator blocks 102, 104, 106, 108 are, therefore, energized directly beneath shuttle 110 and one block ahead of shuttle 110 so that the force is relatively constant as shuttle 110 traverses the track. The switching of applied voltage along sequential motor blocks is termed block switching. Some linear motor systems use a set of discrete sensors such as optical or hall effect sensors in order to determine the position of the shuttle relative to the motor blocks so that the proper motor blocks can be energized. Typically, there would be one or two of these sensors per motor block since only a crude measurement is required. For vector current control and for precise motion control, a finer measure of position is generated from a device such a linear encoder 112 (or equivalent). These might typically sense position down to a fraction of an electrical cycle. Encoder position feedback, stator current feedback, voltage feedback, and in some cases coarse block sensor feedback are available to the control system. A typical control system consists of a processing element such a computer or digital signal processor engine 114 in combination with sensor conditioning and power electronics inverter 116 that provide voltages and currents suitable to operate the linear motor.
The paper titled “Fault Tolerant Operation of Induction Motor Drives with Automatic Controller Reconfiguration”, IEMDC 2001, discloses a reorganizing control strategy of a fault tolerant controller for rotary induction motors. In that system, an encoder failure would be detected and after a force transient due to detection latency, a sensorless controller would be engaged. It was not known when, or if, an encoder failure might occur. That work was targeted to applications where rotary speed would vary slowly so that a sensorless controller could readily be initialized and operated with minimal error. The solution provided a “limp back” capability in which a faulted system could still operate.
One application for a long linear motor is to accelerate an aircraft over a several hundred feet track and then to stop almost immediately after the aircraft has been launched. FIG. 2 shows a typical launch profile and identifies particularly difficult operating conditions at the start and end of the launch. At the start of the launch at near zero speed and high force, there is insufficient information in the sensed voltages and currents for a measurement based sensorless controller and in fact, recent studies have found the motor to be unobservable at this point. If the motor is started without feedback control so that a sensorless controller could be engaged at a higher speed, there would be a period of time required for the sensorless controller to properly lock onto the motor profile. This would create force transients and speed variations that would jeopardize the ability of the system to properly attain the required end speed and keep jerk within acceptable limits in order to launch the aircraft. At the point that the aircraft is released, there is a very large mass change as the plane is disengaged from the motor shuttle and there is a requirement for sudden braking in order to stop the shuttle itself from traveling beyond the track. This region of operation is also not amenable to standard sensorless control techniques. Neither of these conditions is addressed in the previous work.
Whatever the precise merits, features, and advantages of such prior art linear motor systems, none of them achieves or fulfills the purposes of the present invention.